1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display and fabricating method thereof that can improve productivity and yield.
2. Discussion of the Related Art
A liquid crystal display controls the light transmittance of liquid crystal by use of electric field, thereby displaying a picture. To this end, the liquid crystal display includes a liquid crystal display panel in which a plurality of liquid crystal pixel cells are arranged in a matrix at each crossing of data lines and gate lines, and a drive circuit for driving the liquid crystal display panel.
Generally, the liquid crystal display panel includes a thin film transistor substrate and a color filter substrate which face each other; liquid crystal injected between the two substrates; and a spacer for keeping a cell gap between the two substrates.
The thin film transistor substrate includes a pixel electrode which is formed at each of liquid crystal pixel cell areas defined by the crossing of the gate line and the data line; a thin film transistor connected to the gate line, the data line and the pixel electrode; a plurality of insulating films; and an alignment film formed for aligning the liquid crystal in an initial direction.
The color filter substrate includes a color filter formed at each liquid crystal pixel cell; a black matrix for dividing between color filters and blocking external light; a common electrode for supplying a reference voltage to the liquid crystal; and an alignment film for aligning the liquid crystal thereon.
The thin film transistor substrate and the color filter substrate are bonded together, and then the liquid crystal is injected and sealed to complete the liquid crystal display panel. Alternatively, the liquid crystal is spread over any one of the two substrates, and then the two substrates are bonded together to complete the liquid crystal display panel. At this moment, the color filter substrate and the thin film transistor substrate are bonded together by aligning the color filter of the color filter substrate and the pixel electrode of the thin film transistor substrate in a one-to-one relationship.
To this end, an align mark is formed for bonding together the thin film transistor substrate and the color filter substrate. The align mark of the thin film transistor substrate is formed together with a gate electrode in a process of forming the gate electrode on a lower substrate. And, the align mark of the color filter substrate is formed together with the black matrix in a process of forming the black matrix on an upper substrate.
When bonding together the thin film transistor substrate and the color filter substrate, if there is misalignment between the thin film transistor substrate and the color filter substrate, a light leakage defect can be generated in the liquid crystal display. In order to prevent such a problem, there is a method in which the width of the black matrix of the color filter substrate is wider, but this method degrades the aperture ratio of the liquid crystal display.
Accordingly, there has recently been proposed a color filter on thin film transistor (hereinafter, referred to as “COT”), where the color filter is formed on the thin film transistor substrate.
FIG. 1 is a cross sectional view illustrating a liquid crystal display of a COT structure according to the related art.
Referring to FIG. 1, the liquid crystal display of the COT structure according to the related art includes a thin film transistor substrate (hereinafter, referred to as a “lower plate”) and a upper plate. The lower plate has a thin film transistor on a lower substrate 1; a color filter 34 for realizing R, G, B pixels formed on the thin film transistor and a black matrix 32; a pixel electrode 22 overlapped with the color filter 34 with an overcoat layer 52 therebetween; and a lower alignment film 38 for aligning liquid crystal. The upper plate has a common electrode 24 which supplies a common voltage to the liquid crystal pixel cells; and a upper alignment film 36 on an upper substrate 2 for aligning liquid crystal.
The thin film transistor includes a gate electrode 6 connected to a gate line (not shown), a source electrode 8 connected to the data line 4, and a drain electrode 10 connected to the pixel electrode 22. Further, the thin film transistor includes an active layer 14 which overlaps the gate electrode 6 with a gate insulating film 12 therebetween to form a channel between the source electrode 8 and the drain electrode 10; and an ohmic contact layer 16 for reducing a contact resistance between the active layer 14 and the source and drain electrodes 8 and 10.
A passivation film 18 is formed on the gate insulating film to cover the thin film transistor and the data line 4.
The color filter 34 divided by pixel area is formed on the passivation film 18.
The black matrix 32 is formed to overlap the thin film transistor and to be laid over the adjacent color filters 34 along the gate line and the data line 4 on the passivation film 18 where the color filter 34 is formed. The black matrix 32 prevents an interference between the color filters 34, an external light reflection by the metallic gate and data bus line, and a light leakage current caused when a channel part of the thin film transistor 30 is exposed to the external light.
An overcoat layer 52 made of an organic insulating material is formed on the color filter 34 and the black matrix 32. The overcoat layer 52 provides an even surface by compensating a step difference between the color filter 34 and the black matrix 32, and prevents foreign materials of the color filter 34 and the black matrix 32 from flowing into the liquid crystal.
The pixel electrode 22 is independently fowled at each of the pixel areas so as to overlap the color filter 34 on the overcoat layer 52. The pixel electrode 22 is connected to the drain electrode 10 that is exposed through a contact hole 20 that penetrates the overcoat layer 52, the color filter 34 and the passivation film 18.
The upper and lower alignment films 36 and 38 for aligning liquid crystal are formed by performing a rubbing process after spreading an alignment material such as polyimide on the upper and lower substrates 1 and 2.
The lower plate and the upper plate are bonded together with a gap, the liquid crystal is then injected between the gap, and then they are sealed so as to complete the liquid crystal display panel. Alternatively, the liquid crystal is spread over any one of the two substrates, and then the two substrates are bonded together to complete the liquid crystal display panel.
In order to bond the lower and upper plates together, the liquid crystal display includes a lower align mark 26 on the lower substrate 1, and a upper align mark 28 on the upper substrate 2. The lower align mark 26 and the upper align mark 28 are removed in a scribing process after the bonding process.
The lower align mark 26 is formed together with the gate electrode 6 or the black matrix 32 on the lower substrate 1 in a process of forming the gate electrode 6 or the black matrix 32. However, the upper alignment mark 28 is formed on the upper substrate 2 by an extra photolithography process using an opaque metal material different from the common electrode 24. The common electrode 24 is formed of a transparent conductive material in a photolithography process. Accordingly, when fabricating the liquid crystal display of the COT structure, an additional photolithography process is required to form the upper alignment mark 28 on the upper substrate 2, thereby decreasing the productivity of the liquid crystal display of the COT structure.